Automotive industry is undergoing a rapid evolution with road vehicles continuously providing innovative technologies and services such as autonomous driving and interconnection with other vehicles. These technological novelties are constantly increasing the number of embedded electronics in vehicles. However, no equal growth is achieved in the development of modern protocols that will provide essential security measures. Control Area Network (CAN) bus and the underlying bus protocol used for the communication among the different vehicle Electronic Control Units (ECU) on a vehicular CAN network, was designed in an era of very limited security awareness; however, it still is the dominant protocol used in road vehicles. The lack of security features on CAN bus has led to a notable increase in attacks against road vehicles that usually lead to a total compromise. In the present thesis we examine a diagnostic protocol using reverse engineering methodologies and analyze how it works, as well as how it is used to perform various actions on a real car. Furthermore, by utilizing our reverse engineering findings, we show how custom software can be developed, that allows to trigger pre-programmed actions in the car and perform reconnaissance in the internal car network and the Electronic Control Units residing on it.